1. Field of the Invention
The present invention relates generally to a mobile communication system using a retransmission scheme, and in particular, to an apparatus and method for performing soft symbol combining in a receiver.
2. Description of the Related Art
In general, a receiver performs soft symbol combining to improve reception performance in a mobile communication system using a retransmission scheme (e.g., HARQ: Hybrid Automatic Repeat reQuest).
FIG. 1 illustrates data retransmission using a different modulation scheme and a different code rate for each transmission in a communication system using a retransmission scheme. Referring to FIG. 1, a transmitter transmits data using 16QAM (Quadrature Amplitude Modulation) with a code rate (R) of 1/4 at a first transmission. If a receiver fails to receive the data, it transmits an NACK (Non-Acknowledgement) signal to the transmitter, requesting retransmission of the data. The transmitter then transmits to the receiver additional redundancy information using a different modulation, QPSK (Quadrature Phase Shift Keying) with a code rate of 1/2 at a second transmission (i.e., a first retransmission). The receiver receives the redundancy information and subjects it to soft symbol combining. If the retransmission scheme is CC (Chase Combining), the transmitter transmits the same redundancy information at the retransmission as at the initial transmission. If retransmission scheme is IR (Incremental Redundancy), the transmitter transmits the same or different redundancy information at the retransmission.
Soft Symbol Combining is Performed in Two Ways.
(1) Homogeneous modulation scheme-based soft symbol combining: when initial transmission and retransmission are carried out using the same modulation scheme, the receiver combines soft symbols output from a single demodulator irrespective of initial transmission or retransmission. This soft symbol combining method is adopted in an HARQ system using one modulation_scheme or an HARQ system, which uses a plurality of modulation schemes but maintains the same modulation scheme from an initial transmission of data to the last retransmission of the data.
(2) Heterogeneous modulation scheme-based soft symbol combining: when initial transmission and retransmission are carried out using different modulation schemes, the receiver combines soft symbols output from different demodulators at the initial transmission and retransmission. This soft symbol combining method is adopted in a communication system using an AMCS (Adaptive Modulation and Coding Scheme) and an adaptive HARQ.
The homogeneous modulation scheme and the heterogeneous modulation scheme will be described below in more detail.
FIGS. 2 and 3 are block diagrams of conventional receivers operating according to the homogeneous modulation scheme and according to the heterogeneous modulation scheme, respectively. It is assumed here that a transmitter selects one modulation scheme in an HARQ algorithm at each data transmission. The operation of the transmitter depends on system implementation, and thus its description is not provided here.
Referring to FIGS. 2 and 3, the receivers obtain soft metrics (soft outputs) from demodulators 201-1 and 202-1 (hereinafter, collectively referred to as 201) and demodulators 202-1 and 202-2 (hereinafter, collectively referred to as 202) that operate according to different modulation schemes used for initial transmission and retransmission. Soft symbol controllers 203-1 and 203-2 (hereinafter, collectively referred to as 203) output the arithmetic sum of the soft metrics as a combined soft symbol metric (output) when soft symbol combining is used. The soft symbol controllers 203 rearrange the soft symbols in the original order when soft symbol combining is not used. Though not illustrated, normalization blocks may be disposed between the demodulators 201 and 202 and turbo decoders 204-1 and 204-2 (hereinafter, collectively referred to as 204) in order to prevent overflow caused by fixed-point operation. Normalization can be performed in many well-known methods and thus it will not be described here.
Because the receiver illustrated in FIG. 2 operates according to the homogeneous modulation scheme, the demodulators 201-1 and 202-1 support the same modulation scheme, QPSK. On the other hand, since the receiver illustrated in FIG. 3 operates according to the heterogeneous modulation scheme, the demodulators 201-2 and 202-2 support different modulation schemes, QPSK and 16QAM, respectively. The demodulators 201 demodulate initial transmission data and the demodulators 202 demodulate retransmission data. When the soft symbol controllers 203 start to operate, the outputs of the demodulators 201 and 202 are provided simultaneously. While the receiver supports two modulation schemes in FIG. 3 for illustrative purposes, it can be expanded to support more modulation schemes by using more demodulators.
The demodulators illustrated in FIGS. 2 and 3 can output soft metrics in many ways (e.g., by DMM (dual minimum metric) provided in “Evaluation Methodology”, a system simulation guidebook presented by the 3GPP2 (3rd Generation Partnership Project 2), or by maximum likelihood metric to minimize errors). The present invention can be implemented with use of any soft metric generation method. Here, DMM will be adopted by way of example.
In the conventional soft symbol combining method as illustrated in FIGS. 2 and 3, irrespective of modulation schemes used for an initial transmission and retransmissions, a soft metric is obtained independently according to a corresponding modulation scheme for each transmission. Then the soft symbol controllers 203 operate differently, depending on whether soft symbol combining is used or not. Specifically, the two demodulators 201 and 202 directly feed soft metrics to the soft symbol controllers 203. If soft symbol combining is used, the soft symbol controllers 203 arithmetically calculate the average of the two soft metrics. If soft symbol combining is not used, the soft symbol controllers 203 rearrange soft symbols.
In view of the nature of turbo codes and the performance of a MAP (Maximum A Posteriori) decoder, a LogMAP decoder, and a MaxLogMAP decoder used as the turbo decoders 204, the following considerations must be taken into account in the above direct soft metric feeding from demodulators using different modulation schemes to a turbo decoder.
In general, a channel reliability Lc is multiplied with an input soft metric during turbo decoding in a turbo decoder 204 illustrated in FIG. 2 or 3. In an AWGN (Additive White Gaussian Noise) environment, the channel reliability Lc=4Eb/No and increases proportionally to the SNR (Signal-to-Noise Ratio) of the channel. Thus, representing the reliability of the soft symbol, the channel reliability Lc is a kind of weighting factor that varies with the quality of the soft symbol when the turbo decoder 204 performs MAP decoding. If the SNR changes, therefore, this weighting factor is changed. Because the target SNR of the AWGN channel is preset, the channel reliability Lc is determined according to the target SNR.
The channel reliability Lc is a very significant factor to the MAP decoder. It is well known that an estimation error (e.g., overestimation or underestimation) of the channel reliability Lc greatly degrades decoding performance. To minimize the performance degradation, a MaxLogMAP decoder is used instead of the MAP decoder.
If a signal processed by the homogenous modulation scheme is transmitted on a static channel such as AWGN, there is no change in the channel reliability Lc. Therefore, the soft symbol controller 203-1 simply calculates the average of soft metrics from the demodulators 201-1 and 202-1 in the receiver illustrated in FIG. 2 because the channel reliability of the soft metrics are identical.
In the heterogeneous modulation scheme, however, the channel reliability L, is different for different modulation schemes. For example, if a QPSK soft metric is combined with an 8PSK or 16QAM soft metric, the modulation schemes have different channel reliabilities. In order to optimize decoding performance, the soft metrics must be weighted according to the channel reliabilities of the modulation schemes for the same reason that the channel reliabilities Lc are weighted according to channel condition in a turbo decoder as stated above.
When a 16QAM soft metric is combined with a QPSK soft metric in actual implementation, weighting the soft metrics at a ratio of 1:3 (16QAM:QPSK) produces an about 0.8 dB gain increase which might not otherwise be obtained. Here, combining covers both soft symbol combining and symbol rearrangement. Consequently, soft metrics must be weighted according to channel reliability when they are from heterogeneous demodulators. In fact, the conventional symbol combining method focuses mainly on symbol combining of homogenous symbols. Even when it deals with symbol combining of heterogeneous symbols, the same channel reliability Lc is simply applied without weighting during turbo decoding. As a result, decoding performance is degraded.